expose more image info to the portal UI

At minimum, it seems we should add a description field to the portal image creation dialogue. Users just get copies of the underlying image description by default, and have no way to modify it, I believe.

Maybe we should allow users to toggle the global bit on/off?

Finally, given that I'm now creating a bunch of docker format images via the portal, it would be nice for me if the image format was displayed in a column on the images.php and list-images.php pages. I realize that is wasted space most of the time, so maybe it could be an optional column that is only displayed if there are images of more than one format in the list.